An accurate and reliable technique for continuously measuring blood pressure involves inserting a saline filled catheter through the patient's vascular system to the point at which it is desired to perform the measurements. The catheter is connected to a pressure sensor, which measures the pressure in the vessel. An alternative method uses a catheter with a pressure sensor at the tip that directly senses the blood pressure. Procedures such as these are commonly referred to as “invasive procedures” because they involve making an incision through the patient's skin and inserting the catheter into a blood vessel. A problem with invasive procedures is that they can cause patient discomfort and increase the risk of complications such as infection.
Non-invasive blood pressure (NIBP) algorithms typically inflate a pressure cuff above the patient's systolic pressure and measure oscillations under the cuff as the cuff is deflated either in steps or continuously. The resulting oscillometric envelope is used to determine the patients' blood pressure. The cuff pressure corresponding to the maximum oscillation amplitude is typically taken as the mean arterial pressure (MAP). Systolic and Diastolic pressures are computed using a fixed ratio of the maximum oscillation amplitude. Some NIBP monitors also use the shape of the oscillometric envelope to compute the Systolic and Diastolic pressures. The problem with conventional NIBP techniques is that they do not compensate for arterial compliance changes and are therefore imprecise.